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LBI-38861C
DCS and CTCSS signals from the recovered audio are
removed by the high pass filter. The high pass filter is an
8-pole active filter that comprises IC410 and associated
components. The de-emphasis is provided by resistor R451
and capacitor C471. The de-emphasized audio signal is fed
to the audio amplifier on the RX/TX printed circuit board
through the flexible PCB 1. The microcontroller controls the
audio signal to the audio amplifier. When Q408 is on, the
audio signal to the audio amplifier is disabled. When Q408
is off, the audio signal to the audio amplifier is enabled.
IC4 is the audio amplifier. The audio signal at flexible
PCB 1, connection 12 is passed to IC4-3 through variable
resistor VR3 located on the front panel. The gain of the
amplifier is set by resistor R27 and C99. The amplified audio
signal at IC4-5 is applied to the internal speaker by flexible
PCB 1, (connection 9), CON 406 (connection 1) and CON
404 (connection 8). The external speaker connection is
through the 6-way connector.
Mute (Squelch) Circuit
The squelch circuit switches off the audio power ampli-
fier in the absence of audio signals. The squelch circuit
comprises a 25 kHz pass band filter, squelch control VR201
and a noise detector circuit.
The audio signal from IC3-9 is filtered by the 25 kHz
bandpass filter formed by L13, L14, C104 and C105. The
noise in the IF pass band is accepted; frequencies in the voice
Table 1 - Microcontroller Signal Descriptions
ENABLE
SIGNAL NAME
STATE
LOW BATT
L
BATTERY LOW DETECTOR
MONITOR
L
MONITOR SWITCH DETECTOR
PTT
L
PTT SWITCH DETECTOR
CALL
H
CALL DETECTOR
BUSY
L
SQUELCH DETECTOR
L/D
H
LOCK DETECTOR
TXLED
H
RED LED CONTROLLER
CALL LED
H
GREEN LED CONTROLLER
386 EN
H
386 MUTE CONTROLLER
TXEN
L
TX ENABLE CONTROLLER
P/S
H
POWER SAVING CONTROLLER
6
frequency band are rejected. Any noise present at the output
of the filter is applied to the noise detect circuit.
The noise detect circuit in conjunction with IC3 consists
of transistors Q201 and Q202, thermistor TH201 and diode
D205. Any noise signal present is applied to pin 2 of the SQ
board from IC3-11. The signal is amplified by Q201, recti-
fied by D201 then buffered by Q202. The buffered signal is
applied to VR201 for adjustment of squelch sensitivity and
applied through R149 to the squelch trigger at IC3-12.
When noise is present, the voltage at IC3-12 exceeds 0.7
V, the squelch trigger output is 0 V (logic 0), muting the
receiver audio circuit. When no noise is present, the voltage
at IC3-12 is less than 0.7 V and IC3-13 is at 5V (logic 1),
unmuting the receiver audio circuit. Resistor R119 is used to
provide hysteresis of 3 to 6 dB.
MICROCONTROLLER
Microcontroller, IC409, is a Zilog based Z86C10 with
4k-byte ROM and 128k-byte RAM. It has a clock frequency
of 11.1552 MHz derived from crystal X401 which is con-
nected between pins 30 and 31. The microcontroller, in
conjunction with control signals from the switches on the top
panel, controls the frequency synthesizer circuit from data
stored in the EEPROM, the control and indicator circuit, the
Channel Guard circuit, and the power save circuit.
Table 1 identifies the microcontroller interface signals.
DESCRIPTION/FUNCTION
Table 1 - Microcontroller Signal Descriptions (Cont.)
ENABLE
SIGNAL NAME
STATE
R/E
H
PLL SELECTOR (MC 145156)
E/E
H
EEPROM SELECTOR (93C46)
E/E
H
EEPROM SELECTOR (93C46)
XS/E
H
X-SW SELECTOR (MC 142100)
1
DATA
EEPROM CLK/X-SW A/TONE A/CH SW A(1)
2
DATA
X-SWB/TONE B/CHANNEL SW B(2)
4
DATA
EEPROM D1/X-SW D/CH SW C(4)/PLL CLOCK
8
DATA
X-SW C/TONE C/CH SW D(8)
D
DATA
EEPROM DO/X-SW DATA/PLL DATA
CALLDET
INTERUPT
CALL DETECTOR INTERRUPT
I/O EN
L
I/O CONTROL SELECTOR (74HC368)
CLOCK
CLK
FILTERING CLOCK GENERATOR (MF 6-100)
Receive And Transmit Switching Circuit
The receive and transmit switching circuit in conjunction
with microcontroller IC409 consists of transistors Q4, Q5
Q401, diode D9 and IC402. They are configured so that when
a switch in the receiver circuit is closed the transmitter circuit
is off.
When the PTT switch (SW-402) on flexible PCB 9 is
pressed, the PTT input at IC409-43 is held low, causing trans-
mit enable pin (TX EN) of IC409 to go low (0 V) and power
save pin (P/S) to go high (+5V). TX EN is connected to Q401,
IC402 on the control printed circuit board, and TX switching
transistor Q4 and D9 on the RF printed circuit board.
Analog switch IC402A is disabled and Q401 is forward
biased, applying +5V to IC404 and analog switch IC402B. This
enables the microphone audio circuit. The transmit enable
signal is passed to Q4 through CON401, connection 7 and
flexible PCB 1, connection 6.
CTCSS/DCS Tone Encode And Decode
Circuits
The CTCSS/DCS (tone/digital) encode and decode cir-
cuits, in conjunction with IC409, comprise an addressable 4 x
DESCRIPTION/FUNCTION
4 cross point switch (IC406) and a 6th order switched capacitor
lowpass filter IC407). IC407 has two internal general purpose
op-amps. When programmed the circuit is used to control the
audio muting function of the radio. Table 2 details the 4 x 4
cross point switch input/output descriptions. The cross point
switching is detailed in Table 3.
When the CTCSS/DCS decoding circuit IC406 is enabled,
CTCSS/DCS signals present at CON402, connection 15 are
passed to IC407-8 (FIL IN) through R4 (IC406-11), C3
(IC406-12) and capacitor C436. The clock frequency is used
to set the cut-off frequency to filter out the CTCSS/DCS tone
frequency. The filtered tone frequency at IC407-3 is passed to
the input of an internal amplifier through pin 13. During
CTCSS decoding operation the filtered tone frequency at Pin
3 (IC407) is passed to the CTCSS filter through the coupling
cap C455 to IC412-3. The CTCSS signal is then output from
IC421-1 through coupling cap C454 to IC407-13. The output
of the internal amplifier (IC407-2) is fed to inverter input
IC411-12. The inverted output IC411-11 is applied to the
CALL pin and CALL DET pin 2B of the microcontroller. The
signal is filtered and matched with a pre-programmed fre-
quency and if successful the TX LED and BUSY LED output
lines are set a +5V. The green LED on the front panel is turned
on and the receiver audio circuit is enabled. For DCS, R2 of
IC406 and C4 are added to improve low pass filtering.
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